1. Field of the Invention
The present invention relates to a process for operating a coal-fired furnace. The present invention also relates to a process for operating a coal-fired furnace with reduced slag formation. The present invention further relates to a method for reducing slag formation in a coal-fired furnace.
2. Description of the Related Art
Slag builds up on the surfaces and/or walls of furnaces and boilers due to deposition of molten and/or semi-molten ash, which can in turn solidify. Particles of ash are normally molten when they exit the flame zone or radiant section of a boiler or furnace (the terms “furnace” and “boiler” are used interchangeably herein). If the melting point of the ash or the rate of solidification is too low, the particles will not have sufficient time to solidify before impinging on or contacting a surface within the boiler or furnace. When this occurs, the molten or plastic-like ash adheres to and solidifies on the surface, which gives rise to a slag deposit. Fouling can also occur in lower temperature convective sections of the boiler or furnace when volatile components in the ash, such as the alkali oxides, condense and collect further ash, which can sinter into a hard mass.
Typically, the composition and physical properties of ash found in prospective coal feedstocks are considered when designing the size and thermal dynamics of a boiler or furnace. Slag formation can be a particular problem when a coal feedstock is used in a boiler or furnace for which the boiler or furnace was not designed. The size and thermal dynamics of the boiler relative to the composition and physical properties of the ash in the coal feedstock will determine whether the ash is solid or molten by the time it reaches a surface. Desirably, the boiler or furnace is designed such that ash solidifies prior to reaching surfaces within the boiler or furnace. Such solidified ash can be removed relatively easily by means known in the art, such as by physical removal or blowing.
Slag formation occurs to some extent in all boiler and furnace systems. Boilers are often designed for some slag buildup on surfaces and walls to provide an additional measure of thermal insulation, and, thus, minimize heat loss through the walls. Excessive slag buildup, however, tends to clog the boiler or furnace and/or result in excessive temperatures therein.
Slag formation can have a major impact on boiler operation. Significant accumulation of slag can result in partial blockage of the gas flow, possibly requiring reduction in boiler load. Slag may build up to an extent that damage to tubing may result when attempting to dislodge heavy accumulations. Insulation of waterwall tubes may lead to a thermal imbalance within the boiler, heat transfer efficiency reductions, and excessively high temperatures in the superheat section.
Boilers are generally designed around a specified range of coal properties, depending on the expected source of fuel. Many consumers are forced to switch their normal supplies because of increased demand for coal. Additionally, more stringent regulations regarding emissions may make a change in fuel more desirable than adding control systems. Alternate coal supplies may be completely different from design fuel with regard to ash fusion temperature, ash composition, etc. Substitution of a coal with ash characteristics significantly different from those for which a boiler was designed can give rise to problems such as slagging.
Many factors are considered in designing a boiler capable of handling the ash characteristics of a particular coal. Design considerations are very important in determining whether deposits will form when a particular fuel is burned. Design considerations are geared to optimize the combustion process and reduce deposits to a minimum thus maximizing the efficiency of extraction of energy from the fuel. Careful control of the relative quantities absorbed through the various boiler sections is necessary.
A method commonly used in the art to reduce slag formation during on-line operations is soot blowing. However, soot blowing usually only partially alleviates the problem of slag formation.
Another method of reducing slag formation while on-line is to reduce boiler or furnace load. During reduction of boiler load, temperatures are reduced and molten ash solidifies faster, i.e., prior to reaching boiler/furnace walls. Also, the temperature reduction can cause a difference in contraction rates between metal in the tubes and the slag and cause slag to be separated from tube surfaces. Notwithstanding the foregoing, reduction of boiler load is economically undesirable due to lost capacity.
Another method used in the art to reduce slag formation while on-line is the use of attemperating spray, which reduces steam temperatures. As tubes begin to encounter slag formation, excessively high steam temperatures in the superheat and/or reheat sections of the boiler or furnace may necessitate the use of an attemperating spray. If slagging continues to increase, the amount of spray must be increased. Since the level of attemperating spray usage is proportional to the degree of slag formation, it can serve as a useful measure of the severity of the slag formation. When maximum spray is reached and steam temperatures are still too high, thermal balance can be restored by reducing load and shedding or removing slag.
It would be desirable to have a process for operating a coal-fired furnace exhibiting reduced slag formation. It would also be desirable to have a method for reducing slag formation in a coal-fired furnace.